Multi-Layer steel cable for tire carcass

ABSTRACT

A multi-layer cable having an unsaturated outer layer, usable as a reinforcing element for a tire carcass reinforcement, comprising a core (C 0 ) of diameter d 0  surrounded by an intermediate layer (C 1 ) of six or seven wires (M=6 or 7) of diameter d 1  wound together in a helix at a pitch P 1 , this layer C 1  itself being surrounded by an outer layer (C 2 ) of N wires of diameter d 2  wound together in a helix at a pitch P 2 , N being less by 1 to 3 than the maximum number N max  of wires which can be wound in one layer about the layer C 1 , this cable being characterised in that it has the following characteristics (d 0 , d 1 , d 2 , p 1  and p 2  in mm):  
     (i) 0.14&lt;d 0 &lt;0.28;  
     (ii) 0.12&lt;d 1 &lt;0.25;  
     (iii) 0.12&lt;d 2 &lt;0.25;  
     (iv) for M=6: 1.10&lt;(d 0 /d 1 )&lt;1.40; for M=7: 1.40&lt;(d 0 /d 1 )&lt;1.70;  
     (v) 5π(d 0 +d 1 )&lt;p 1 &lt;p 2 &lt;5π(d 0 +2d 1 +d 2 );  
     (vi) the wires of layers C 1  and C 2  are wound in the same twist direction.  
     The invention furthermore relates to the articles or semi-finished products made of plastics material and/or rubber which are reinforced by such a multi-layer cable, in particular to tires intended for industrial vehicles, more particularly truck tires and their carcass reinforcement plies.

[0001] The present application is a continuation of InternationalApplication No. PCT/EP 00/05822, filed Jun. 26, 2000, published inFrench with an English Abstract on Jan. 4, 2001 under PCT Article 21(2),which claims priority to French Patent Application No. 99/08446, filedJun. 29, 1999.

[0002] The present invention relates to steel cables (“steel cords”)which can be used for reinforcing rubber articles such as tires. Itrelates more particularly to the cables referred to as “layered” cableswhich can be used for reinforcing the carcass reinforcements of tiresfor industrial vehicles such as “truck” tires.

[0003] Steel cables for tires, as a general rule, are formed of wires ofperlitic (or ferro-perlitic) carbon steel, hereinafter referred to as“carbon steel”, the carbon content of which is generally between 0.2%and 1.2%, the diameter of these wires most frequently being between 0.10and 0.40 mm (millimeters). A very high tensile strength is required ofthese wires, generally greater than 2000 MPa, preferably greater than2500 MPa, which is obtained owing to the structural hardening whichoccurs during the phase of work-hardening of the wires. These wires arethen assembled in the form of cables or strands, which requires thesteels used also to have sufficient ductility in torsion to withstandthe various cabling operations.

[0004] For reinforcing carcass reinforcements of truck tires, nowadaysmost frequently so-called “layered” steel cables (“layered cords”) or“multi-layer” steel cables formed of a central core and one or moreconcentric layers of wires arranged around this core. These layeredcables, which favor greater contact lengths between the wires, arepreferred to the older “stranded” cables (“strand cords”) owing firstlyto greater compactness, and secondly to lesser sensitivity to wear byfretting. Among layered cables, a distinction is made in particular, inknown manner, between compact-structured cables and cables havingtubular or cylindrical layers.

[0005] The layered cables most widely found in the carcasses of trucktires are cables of the formula (L+M) or (L+M+N), the latter generallybeing intended for the largest tires. These cables are formed, in knownmanner, of a core of L wire(s) surrounded by at least one layer of Mwires which may itself be surrounded by an outer layer of N wires, withgenerally L varying from 1 to 4, M varying from 3 to 12, N varying form8 to 20; if applicable, the assembly may possibly be wrapped by anexternal wrapping wire wound in a helix around the last layer.

[0006] Such layered cables which can be used for reinforcing tirecarcasses, in particular carcasses of truck tires, have been describedin a very large number of publications. Reference will be made inparticular to the documents U.S. Pat. Nos. 3,922,841; 4,158,946;4,488,587; EP-A-0 168 858; EP-A-0 176 139 or U.S. Pat. No. 4,651,513;EP-A-0 194 011; EP-A-0 260 556 or U.S. Pat. No. 4,756,151; EP-A-0 362570; EP-A-0 497 612 or U.S. Pat. No. 5,285,836; EP-A-0 568 271; EP-A-0648 891; EP-A-0 669 421 or U.S. Pat. No. 5,595,057; EP-A-0 709 236 orU.S. Pat. No. 5,836,145; EP-A-0 719 889 or U.S. Pat. No. 5,697,204;EP-A-0 744 490 or U.S. Pat. No. 5,806,296; EP-A-0 779 390 or U.S. Pat.No. 5,802,829; EP-A-0 834 613; WO98/41682; RD (Research Disclosure) No.34054, August 1992, pp. 624-33; RD No. 34370, November 1992, pp. 857-59.

[0007] In order to fulfil their function as reinforcement for tirecarcasses, the layered cables must first of all have good flexibilityand high endurance under flexion, which implies in particular that theirwires are of relatively low diameter, normally less than 0.28 mm,preferably less than 0.25 mm, in particular less than that of the wiresused in conventional cables for crown reinforcements of tires.

[0008] These layered cables are furthermore subjected to major stressesduring running of the tires, in particular to repeated flexure orvariations in curvature, which cause friction at the level of the wires,in particular as a result of the contact between adjacent layers, andtherefore of wear, and also of fatigue; they must therefore have highresistance to so-called “fatigue-fretting” phenomena.

[0009] Finally, it is important for them to be impregnated as much aspossible with rubber, and for this material to penetrate into all thespaces between the wires forming the cables, because if this penetrationis insufficient, there then form empty channels along the cables, andthe corrosive agents, for example water, which are likely to penetrateas far as into the tires for example as a result of cuts, move alongthese channels and into the carcass of the tire. The presence of thismoisture plays an important part in causing corrosion and inaccelerating the above degradation processes (so-called“fatigue-corrosion” phenomena), compared with use in a dry atmosphere.

[0010] All these fatigue phenomena which are generally grouped togetherunder the generic term “fatigue-fretting-corrosion” are at the origin ofgradual degeneration of the mechanical properties of the cables, and mayadversely affect the life thereof under very severe running conditions.

[0011] In order to improve the endurance of layered cables in truck tirecarcasses, in which in known manner the repeated flexural stresses maybe particularly severe, it has for a long time been proposed to modifythe design thereof in order to increase, in particular, their ability tobe penetrated by rubber, and thus to limit the risks due to corrosionand to fatigue-corrosion.

[0012] There have for example been proposed or described layered cablesof the construction (3+9) or (3+9+15) which are formed of a core of 3wires surrounded by a first layer of 9 wires and if applicable a secondlayer of 15 wires, as described, for example, in EP-A-0 168 858, EP-A-0176 139, EP-A-0 497 612, EP-A-0 669 421, EP-A-0 709 236, EP-A-0 744 490and EP-A-0 779 390, the diameter of the wires of the core being or notbeing greater than that of the wires of the other layers. These cablescannot be penetrated as far as the core owing to the presence of achannel or capillary at the centre of the three core wires, whichremains empty after impregnation by the rubber, and therefore favorableto the propagation of corrosive media such as water.

[0013] The publication RD No. 34370 describes cables of the structure[1+6+12], of the compact type or of the type having concentric tubularlayers, formed of a core formed of a single wire, surrounded by anintermediate layer of 6 wires which itself is surrounded by an outerlayer of 12 wires. The ability of rubber to penetrate it can be improvedby using diameters of wires which differ from one layer to the other, oreven within one and the same layer. Cables of construction [1+6+12], thepenetration ability of which is improved owing to appropriate selectionof the diameters of the wires, in particular to the use of a core wireof larger diameter, have also been described, for example in EP-A-0 648891 or WO98/41682.

[0014] In order to improve further, relative to these conventionalcables, the penetration of the rubber into the cable, there have beenproposed multi-layer cables having a central core surrounded by at leasttwo concentric layers, for example cables of the formula [1+6+N], inparticular [1+6+11], the outer layer of which is unsaturated(incomplete), thus ensuring better penetration ability by the rubber(see, for example, EP-A-0 719 889, WO98/41682). The proposedconstructions make it possible to dispense with the wrapping wire, owingto better penetration of the rubber through the outer layer and theself-wrapping which results; however, experience shows that these cablesare not penetrated right to the centre by the rubber, and in any casenot yet optimally.

[0015] Furthermore, it should be noted that an improvement in theability of the rubber to penetrate is not sufficient to ensure asufficient level of performance. When they are used for reinforcing tirecarcasses, the cables must not only resist corrosion, but also mustfulfil a large number of sometimes contradictory criteria, in particularof tenacity, resistance to fretting, high degree of adhesion to rubber,uniformity, flexibility, endurance under repeated flexing or traction,stability under severe flexing, etc.

[0016] Thus, for all the reasons set forth previously, and despite thevarious recent improvements which have been made here or there on suchand such a given criterion, the best cables used today in carcassreinforcements for truck tires remain limited to a small number oflayered cables of highly conventional structure, of the compact type orthe type having cylindrical layers, with a saturated (complete) outerlayer; these are essentially cables of constructions [3+9], [3+9+15] or[1+6+12] as described previously.

[0017] Now, the Applicant during its research discovered a novel layeredcable of the type having an unsaturated outer layer, which unexpectedlyimproves further the overall performance of the best layered cablesknown for reinforcing truck tire carcasses. This cable of the invention,owing to a specific structure, not only has excellent ability to bepenetrated by the rubber, limiting the problems of corrosion, but alsohas fatigue-fretting endurance properties which are significantlyimproved compared with the cables of the prior art. The longevity oftruck tires and that of their carcass reinforcements is thus verysubstantially improved thereby.

[0018] Consequently, a first subject of the invention is a multi-layercable having a unsaturated outer layer, usable as a reinforcing elementfor a tire carcass reinforcement, comprising a core (C0) of diameter d₀surrounded by an intermediate layer (C1) of six or seven wires (M=6 or7) of diameter d₁ wound together in a helix at a pitch p₁, this layer C1itself being surrounded by an outer layer (C2) of N wires of diameter d₂wound together in a helix at a pitch p_(2,) N being less by 1 to 3 thanthe maximum number N_(max) of wires which can be wound in one layerabout the layer C1, this cable being characterised in that it has thefollowing characteristics (d₀, d₁, d₂, p₁ and p₂ in mm):

[0019] (i) 0.14<d₀<0.28;

[0020] (ii) 0.12<d₁<0.25;

[0021] (iii) 0.12<d₂<0.25;

[0022] (iv) for M=6: 1.10<(d₀/d₁)<1.40; for M=7: 1.40<(d₀/d₁)<1.70;

[0023] (v) 5π(d₀+d₁)<p₁<p₂<5π(d₀+2d₁+d₂);

[0024] (vi) the wires of layers C1 and C2 are wound in the samedirection of twist.

[0025] The invention also relates to the use of a cable according to theinvention for reinforcing articles or semi-finished products made ofplastics material and/or of rubber, for example plies, tubes, belts,conveyor belts and tires, more particularly tires intended forindustrial vehicles which usually use a metal carcass reinforcement.

[0026] The cable of the invention is very particularly intended to beused as a reinforcing element of a carcass reinforcement for a tireintended for industrial vehicles selected from among vans, “heavyvehicles”—i.e. subway trains, buses, road transport machinery (lorries,tractors, trailers), off-road vehicles—agricultural machinery orconstruction machinery, aircraft, and other transport or handlingvehicles.

[0027] The invention furthermore relates to these articles orsemi-finished products made of plastics material and/or rubberthemselves when they are reinforced by a cable according to theinvention, in particular tires intended for the industrial vehiclesmentioned above, more particularly truck tires, and also to compositefabrics comprising a matrix having a composition of rubber compositionreinforced with a cable according to the invention, usable as a carcassreinforcement ply for such truck tires.

[0028] The invention and its advantages will be readily understood inthe light of the description and examples of embodiment which follow,and FIGS. 1 to 3 relating to these examples, which show, respectively:

[0029] a cross-section through a cable of structure [1+6+11] accordingto the invention (FIG. 1);

[0030] a cross-section through a cable of structure [1+6+12] of theprior art (FIG. 2);

[0031] a radial section through a truck tire having a radial carcassreinforcement (FIG. 3).

[0032] I. MEASUREMENTS AND TESTS

[0033] I-1. Dynamometric Measurements

[0034] As far as the metal wires or cables are concerned, themeasurements of breaking load Fm (maximum load in N), of tensilestrength Rm (in MPa) and of elongation at break At (total elongation in%) are carried out under tension in accordance with ISO Standard 6892 of1984. As far as the rubber compositions are concerned, the measurementsof modulus are carried out under tension in accordance with StandardAFNOR-NFT-46002 of September 1988: the nominal secant modulus (orapparent stress, in MPa) is measured in a second elongation (i.e. afteran accommodation cycle) at 10% elongation, referred to as M10 (normalconditions of temperature and humidity in accordance with StandardAFNOR-NFT-40101 of December 1979).

[0035] I-2. Air Permeability Test

[0036] The air permeability test makes it possible to measure a relativeindex of air permeability, “Pa”. It is a simple way of indirectlymeasuring the degree of penetration of the cable by a rubbercomposition. It is performed on cables extracted directly, bydecortication, from the vulcanized rubber plies which they reinforce,and which therefore have been penetrated by the cured rubber.

[0037] The test is carried out on a given length of cable (for example 2cm) as follows: air is sent to the entry of the cable, at a givenpressure (for example 1 bar), and the quantity of air is measured at theexit, using a flow meter; during the measurement, the sample of cable islocked in a seal such that only the quantity of air passing through thecable from one end to the other, along its longitudinal axis, is takeninto account for the measurement. The flow measured is lower, the higherthe amount of penetration of the cable by the rubber.

[0038] I-3. Test of Endurance in the Tire

[0039] The endurance of the cables under fatigue-fretting corrosion isevaluated in carcass plies of truck tires for a very long-durationrunning test.

[0040] For this, truck tires are manufactured, the carcass reinforcementof which is formed of a single rubberised ply reinforced by the cablesto be tested. These tires are mounted on suitable known rims and areinflated to the same pressure (with an excess pressure relative tonominal pressure) with air saturated with moisture. Then these tires arerun on an automatic running machine under a very high load (overloadrelative to the nominal load) and at the same speed, for a given numberof kilometers. At the end of the running, the cables are extracted fromthe tire carcass by decortication, and the residual breaking load ismeasured both on the wires and on the cables thus fatigued.

[0041] Furthermore, tires identical to the previous ones aremanufactured and they are decorticated in the same manner as previously,but this time without subjecting them to running. Thus the initialbreaking load of the non-fatigued wires and cables is measured afterdecortication.

[0042] Finally the breaking-load degeneration after fatigue iscalculated (referred to as ΔFm and expressed in %), by comparing theresidual breaking load with the initial breaking load. This degenerationΔFm is due to the fatigue and wear (reduction in section) of the wireswhich are caused by the joint action of the various mechanical stresses,in particular the intense working of the contact forces between thewires, and the water coming from the ambient air, in other words to thefatigue-fretting corrosion to which the cable is subjected within thetire during running.

[0043] It may also be decided to perform the running test until forceddestruction of the tire occurs, owing to a break in the carcass ply oranother type of damage occurring earlier (for example detreading).

[0044] I-4. Belt Test

[0045] The “belt” test is a known fatigue test which was described, forexample, in applications EP-A-0 648 891 or WO98/41682 mentioned above,the steel cables to be tested being incorporated in a rubber articlewhich is vulcanized.

[0046] The principle thereof is as follows: the rubber article is anendless belt produced with a known rubber-based mixture, similar tothose which are currently used for radial tire carcasses. The axis ofeach cable is oriented in the longitudinal direction of the belt and thecables are separated from the faces of the latter by a thickness ofrubber of about 1 mm. When the belt is arranged so as to form a cylinderof revolution, the cable forms a helical winding of the same axis asthis cylinder (for example, helix pitch equal to about 2.5 mm).

[0047] This belt is then subjected to the following stresses: the beltis rotated around two rollers, such that each elementary portion of eachcable is subjected to a tension of 12% of the initial breaking load andis subjected to cycles of variation of curvature which make it pass froman infinite radius of curvature to a radius of curvature of 40 mm, andthis over 50 million cycles. The test is carried out under a controlledatmosphere, the temperature and the humidity of the air in contact withthe belt being kept at about 20° C. and 60% relative humidity. Theduration of the stresses for each belt is of the order of 3 weeks. Atthe end of these stresses, the cables are extracted from the belts bydecortication, and the residual breaking load of the wires of thefatigued cables is measured.

[0048] Secondly, a belt is manufactured which is identical to theprevious one, and it is decorticated in the same manner as previously,but this time without subjecting the cables to the fatigue test. Thusthe initial breaking load of the wires of the non-fatigued cables ismeasured.

[0049] Finally, the degeneration of breaking load after fatigue iscalculated (referred to as ΔFm and expressed in %), by comparing theresidual breaking load with the initial breaking load.

[0050] This degeneration ΔFm is due in known manner to the fatigue andwear of the wires which are caused by the joint action of the stressesand the water coming from the ambient air, these conditions beingcomparable to those to which the reinforcement cables are subjected inthe tire carcasses.

[0051] I-5. Undulating Traction Test

[0052] The “undulating traction” test is a fatigue test well-known tothe person skilled in the art, in which the material tested is fatiguedin a pure uni-axial extension (extension-extension), that is to saywithout compressive stress.

[0053] The principle is as follows: A sample of the cable to be tested,which is held at each of its two ends by the two jaws of a tractionmachine, is subjected to a tensile or extensional stress, the intensityσ of which varies cyclically and symmetrically (σ_(avg)±σ_(a)) about anaverage value (σ_(avg)), between two extreme values σ_(min)(σ_(avg)−σ_(a)) and σ_(max) (σ_(avg)+σ_(a)) surrounding this averagevalue, at a given ratio of load “R”=(σ_(min)/σ_(max)). The averagestress σ_(avg) is therefore linked to the ratio of load R and to theamplitude σ_(a) by the relationship σ_(avg)=σ_(a)(1+R)/(1−R).

[0054] In practice, the test is performed as follows: a first amplitudeof stress σ_(a) is selected (generally within a range of the order of ¼to ⅓ of the resistance Rm of the cable) and the fatigue test is startedfor a maximum number of 10⁵ cycles (frequency 30 Hz), the load ratio Rbeing set to 0.1. Depending on the result obtained—i.e. breaking ornon-breaking of the cable after this maximum of 10⁵ cycles—a newamplitude σ_(a) is applied (less or greater than the previous one,respectively) to a new test piece, by varying this value σ_(a) inaccordance with the so-called steps method (Dixon & Mood; Journal of theAmerican Statistical Association, 43, 1948, 109-126). Thus a total of 17iterations are effected, the statistical treatment of the tests which isdefined by this steps method resulting in the determination of anendurance limit—σ_(d)—which corresponds to a 50% probability of breakingof the cable at the end of the 10⁵ fatigue cycles.

[0055] For this test, a tensile fatigue machine manufactured by Schenck(Model PSA) is used; the useful length between the two jaws is 10 cm;the measurement is effected in a controlled dry atmosphere (amount ofrelative humidity less than or equal to 5%; temperature 20° C.).

[0056] II. DETAILED DESCRIPTION OF THE INVENTION

[0057] II-1. Cable of the Invention

[0058] The terms “formula” or “structure”, when used in the presentdescription to describe the cables, refer simply to the construction ofthese cables.

[0059] The cable of the invention is a multi-layer cable comprising acore (C0) of diameter d₀, an intermediate layer (C1) of 6 or 7 wires(M=6 or 7) of diameter d₁ and an unsaturated outer layer (C2) of N wiresof diameter d₂, N being less by 1 to 3 than the maximum number N_(max)of wires which can be wound in a single layer around the layer C1.

[0060] In this layered cable of the invention, the diameter of the coreand that of the wires of the layers C1 and C2, the helix pitches (andhence the angles) and the directions of winding of the different layersare defined by all the characteristics cited hereafter (d₀, d₁, d₂, p₁and p₂ expressed in mm):

[0061] (i) 0.14<d₀<0.28;

[0062] (ii) 0.12<d₁<0.25;

[0063] (iii) 0.12<d₂<0.25;

[0064] (iv) for M=6: 1.10<(d₀/d₁)<1.40; for M=7: 1.40<(d₀/d₁)<1.70;

[0065] (v) 5π(d₀+d₁)<p₁<p₂<5 π(d₀+2d₁+d₂);

[0066] (vi) the wires of layers C1 and C2 are wound in the samedirection of twist.

[0067] Characteristics (i) to (vi) above, in combination, make itpossible to obtain, all at once:

[0068] contact forces which are sufficient but limited between C0 andC1, which are beneficial for reduced wear and less fatigue of the wiresof layer C1;

[0069] due in particular to optimisation of the ratio of the diameters(d₀/d₁) and the helix angles formed by the wires of layers C1 and C2,optimum penetration of the rubber through layers C1 and C2 and as far asthe centre C0 of the latter, which firstly ensures very high protectionagainst corrosion or the possible propagation thereof, and secondlyminimal disorganisation of the cable under high flexural stress, notrequiring in particular the presence of a wrapping wire around the finallayer;

[0070] reduced wear by fretting between the wires of layers C1 and C2,despite the presence of different pitches (p₁≠p₂) between the two layersC1 and C2.

[0071] Characteristics (v) and (vi)—different pitches p₁ and p₂, andlayers C1 and C2 wound in the same direction of twist—mean that, inknown manner, the wires of layers C1 and C2 are essentially arranged intwo adjacent, concentric cylindrical (i.e. tubular) layers. So-called“tubular” or “cylindrical” layered cables are thus understood to becables formed of a core (i.e. core part or central part) and one or moreconcentric layers, each tubular in shape, arranged around this core,such that, at least in the cable at rest, the thickness of each layer issubstantially equal to the diameter of the wires which form it; as aresult, the cross-section of the cable has a contour or shell (E) whichis substantially circular, as illustrated for example in FIG. 1.

[0072] The cables having cylindrical or tubular layers of the inventionmust in particular not be confused with so-called “compact” layeredcables, which are assemblies of wires wound with the same pitch and inthe same direction of twist; in such cables, the compactness is suchthat practically no distinct layer of wires is visible; as a result, thecross-section of such cables has a contour which is no longer circular,but polygonal, as illustrated for example in FIG. 2.

[0073] The outer layer C2 is a tubular layer of N wires which isreferred to as “unsaturated” or “incomplete”, that is to say that, bydefinition, there is sufficient space in this tubular layer C2 to add atleast one (N+1)th wire of diameter d₂, several of the N wires possiblybeing in contact with one another. Reciprocally, this tubular layer C2would be referred to as “saturated” or “complete” if there was notenough space in this layer to add at least one (N+1)th wire of diameterd₂.

[0074] Preferably, the cable of the invention is a layered cable ofconstruction [1+M+N], that is to say that its core is formed of a singlewire, as shown, for example, in FIG. 1 (cable referenced C-I).

[0075] This FIG. 1 shows a section perpendicular to the axis (O) of thecore and of the cable, the cable being assumed to be rectilinear and atrest. It can be seen that the core C0 (diameter d₀) is formed of asingle wire; it is surrounded by and in contact with an intermediatelayer C1 of 6 wires of diameter d₁ which are wound together in a helixat a pitch p₁; this layer C1, which is of a thickness substantiallyequal to d₁, is itself surrounded by and in contact with an outer layerC2 of 11 wires of diameter d₂ which are wound together in a helix at apitch p₂, and therefore of a thickness substantially equal to d₂. Thewires wound around the core C0 are thus arranged in two adjacent,concentric, tubular layers (layer C1 of thickness substantially equal tod₁, then layer C2 of thickness substantially equal to d₂). It can beseen that the wires of layer C1 have their axes (O₁) arrangedpractically on a first circle C₁ shown by broken lines, whereas thewires of layer C2 have their axes (O₂) arranged practically on a secondcircle C₂, also shown by broken lines.

[0076] The best compromise of results, with regard in particular to theability of the cable to be penetrated by the rubber and the contactforces between the different layers, is obtained when the followingrelationship is satisfied:

5.3π(d ₀ +d ₁)<p ₁ <p ₂<4.7π(d ₀+2d ₁ +d ₂)  (vii).

[0077] By thus offsetting the pitches and therefore the angles ofcontact between the wires of layer C1 on one hand and those of layer C2on the other hand, the surface area of the channels for penetratingbetween these two layers is increased and the ability of the cable to bepenetrated is improved further, while optimising its fatigue-frettingperformance.

[0078] It will be recalled here that, according to a known definition,the pitch represents the length, measured parallel to the axis O of thecable, at the end of which a wire having this pitch makes a completeturn around the axis O of the cable; thus, if the axis O is sectioned bytwo planes perpendicular to the axis O and separated by a length equalto the pitch of a wire of one of the two layers C1 or C2, the axis ofthis wire (O₁ or O₂) has in these two planes the same position on thetwo circles corresponding to the layer C1 or C2 of the wire in question.

[0079] In the cable according to the invention, all the wires of thelayers C1 and C2 are wound in the same direction of twist, that is tosay in the S direction (“S/S” arrangement) or in the Z direction (“Z/Z”arrangement). Such an arrangement of the layers C1 and C2 is somewhatcontrary to the most conventional constructions of layered cables[L+M+N], in particular those of construction [3+9+15], which mostfrequently require crossing of the two layers C1 and C2 (or an “S/Z” or“Z/S” arrangement) so that the wires of layer C2 themselves wrap thewires of layer C1. Winding the layers C1 and C2 in the same directionadvantageously makes it possible, in the cable according to theinvention, to minimise the friction between these two layers C1 and C2and therefore the wear of the wires constituting them.

[0080] In the cable of the invention, the ratios (d₀/d₁) must be setwithin given limits, according to the number M (6 or 7) of wires of thelayer C1. Too low a value of this ratio is unfavorable to the wearbetween the core and the wires of layer C1. Too high a value adverselyaffects the compactness of the cable, for a level of resistance which isfinally not greatly modified, and its flexibility; the increasedrigidity of the core due to an excessively large diameter d₀ wouldfurthermore be unfavorable to the feasibility itself of the cable duringthe cabling operations.

[0081] The wires of layers C1 and C2 may have a diameter which isidentical or different from one layer to the other. Preferably wires ofthe same diameter (d₁=d₂) are used, in particular to simplify thecabling process and to reduce the costs, as shown, for example, in FIG.1.

[0082] The maximum number N_(max) of wires which can be wound in asingle saturated layer around the layer C1 is of course a function ofnumerous parameters (diameter d₀ of the core, number M and diameter d₁of the wires of layer C1, diameter d₂ of the wires of layer C2). By wayof example, if N_(max) is equal to 12, N may then vary from 9 to 11 (forexample constructions [1+M+9], [1+M+10] or [1+M+11]); if N_(max) is forexample equal to 14, N may then vary from 11 to 13 (for exampleconstructions [1+M+11], [1+M+12] or [1+M+13]).

[0083] Preferably, the number N of wires in the layer C2 is less by 1 to2 than the maximum number N_(max). This makes it possible, in themajority of cases, to form sufficient space between the wires for therubber compositions to be able to infiltrate between the wires of layerC2 and to reach layer C1. The invention is thus preferably implementedwith a cable selected from among cables of the structure [1+6+10],[1+6+11], [1+6+12], [1+7+11], [1+7+12] or [1+7+13].

[0084] By way of examples of preferred cables according to the inventionfor which d₁=d₂, mention will be made in particular of cables whichsatisfy the above relationship (vii) and have the followingconstructions:

[0085] [1+6+10] with d₀=0.15 mm and d₁=d₂=0.13 mm;4.7 mm<p₁<p₂<8 mm;

[0086] [1+6+10] with d₀=0.23 mm and d₁=d₂=0.20 mm; 7.2 mm<p₁<p₂<12.3 mm;

[0087] [1+6+11] with d₀=0.20 mm and d₁=d₂=0.175 mm; 6.2 mm<p₁<p₂<10.7mm;

[0088] [1+6+11] with d₀=0.26 mm and d₁=d₂=0.225 mm; 8.1 mm<p₁<p₂<13.8mm;

[0089] [1+6+12] with d₀=0.26 mm and d₁=d₂=0.20 mm; 7.7 mm<p₁<p₂<12.7 mm;

[0090] [1+6+12] with d₀=0.225 mm and d₁=d₂=0.175 mm; 6.7 mm<p₁<p₂<11.1mm;

[0091] [1+7+11] with d₀=0.25 mm and d₁=d₂=0.175 mm; 7.1 mm<p₁<p₂<11.4mm;

[0092] [1+7+11] with d₀=0.215 mm and d₁=d₂=0.15 mm; 6.1 mm<p₁<p₂<9.8 mm;

[0093] [1+7+12] with d₀=0.23 mm and d₁=d₂=0.155 mm; 6.4 mm<p₁<p₂<10.3mm;

[0094] [1+7+12] with d₀=0.26 mm and d₁=d₂=0.175 mm; 7.2 mm<p₁<p₂<11.6mm;

[0095] [1+7+13] with d₀=0.24 mm and d₁=d₂=0.15 mm; 6.5 mm<p₁<p₂<10.2 mm;

[0096] [1+7+13] with d₀=0.275 mm and d₁=d₂=0.185 mm; 7.7 mm<p₁<p₂<12.3mm.

[0097] The invention is preferably implemented, in the carcasses of thetruck tires, with cables of structure [1+6+N], more preferably ofstructure [1+6+10], [1+6+11] or [1+6+12]. More preferably still, cablesof structure [1+6+11] are used.

[0098] For a better compromise between strength, feasibility andflexural strength of the cable on one hand and ability to be penetratedby the rubber compositions on the other hand, it is preferred that thediameters of the wires of layers C1 and C2, whether or not these wiresare of identical diameters, be between 0.14 and 0.22 mm.

[0099] In such a case, if d₁=d₂, more preferably the followingrelationship is satisfied:

5<p ₁ <p ₂<15.

[0100] For carcass reinforcements for truck tires, the diameters d₁ andd₂ are even more preferably selected between 0.16 and 0.19 mm: adiameter less than 0.19 mm makes it possible to reduce the level of thestresses to which the wires are subjected upon major variations incurvature of the cables, whereas preferably diameters greater than 0.16mm will be selected for reasons in particular of strength of the wiresand of industrial costs.

[0101] When d₁ and d₂ are thus selected to be between 0.16 and 0.19 mm,the following relationships are more preferably satisfied:

0.18<d ₀<0.24;

5<p ₁ <p ₂<12.

[0102] One advantageous embodiment consists, for example, of selectingp₁ to be between 5 and 8 mm and p₂ to be between 8 and 12 mm.

[0103] The invention may be implemented with any type of steel wires,for example carbon steel wires and/or stainless steel wires asdescribed, for example, in the above applications EP-A-0 648 891 orWO98/41682. Preferably a carbon steel is used, but it is of coursepossible to use other steels or other alloys.

[0104] When a carbon steel is used, its carbon content (% by weight ofsteel) is preferably between 0.50% and 1.0%, more preferably between0.68% and 0.95%; these contents represent a good compromise between themechanical properties required for the tire and the feasibility of thewire. It should be noted that, in applications in which the highestmechanical strengths are not necessary, advantageously carbon steels maybe used, the carbon content of which is between 0.50% and 0.68%, and inparticular varies from 0.55% to 0.60%, such steels ultimately being lesscostly because they are easier to draw. Another advantageous embodimentof the invention may also consist, depending on the intendedapplications, of using steels having a low carbon content of for examplebetween 0.2% and 0.5%, owing in particular to lower costs and greaterease of drawing.

[0105] When the cables of the invention are used to reinforced tirecarcasses for industrial vehicles, their wires preferably have a tensilestrength greater than 2000 MPa, more preferably greater than 3000 MPa.In the case of tires of very large dimensions, in particular wireshaving a tensile strength of between 3000 MPa and 4000 MPa will beselected. The person skilled in the art will know how to manufacturecarbon steel wires having such strength, by adjusting in particular thecarbon content of the steel and the final work-hardening ratio (ε) ofthese wires.

[0106] The cable of the invention might comprise an external wrap,formed for example of a single wire, whether or not of metal, wound in ahelix about the cable in a pitch shorter than that of the outer layer,and a direction of winding opposite or identical to that of this outerlayer.

[0107] However, owing to its specific structure, the cable of theinvention, which is already self-wrapped, does not generally require theuse of an external wrapping wire, which advantageously solves theproblems of wear between the wrap and the wires of the outermost layerof the cable.

[0108] However, if a wrapping wire is used, in the general case in whichthe wires of layer C2 are made of carbon steel, advantageously awrapping wire of stainless steel may then be selected in order to reducethe wear by fretting of these carbon steel wires in contact with thestainless steel wrap, as taught by Application WO98/41682 referred toabove, the stainless steel wire possibly being replaced in equivalentmanner by a composite wire, only the skin of which is of stainless steeland the core of which is of carbon steel, as described for example inPatent Application EP-A-0 976 541.

[0109] II-2. Tire of the Invention

[0110] The invention also relates to tires intended for industrialvehicles, more particularly truck tires and also to carcassreinforcement plies for these truck tires.

[0111] By way of example, FIG. 3 shows diagrammatically a radial sectionthrough a truck tire 1 having a radial carcass reinforcement which mayor may not be in accordance with the invention, in this generalrepresentation. This tire 1 comprises a crown 2, two sidewalls 3 and twobeads 4, each of these beads 4 being reinforced with a bead wire 5. Thecrown 2, in known manner, is reinforced by a crown reinforcement 6formed for example of at least two superposed crossed plies, reinforcedby known metal cables. A carcass reinforcement 7 is wound around the twobead wires 5 within each bead 4, the upturn 8 of this reinforcement 7being for example arranged towards the outside of the tire 1, which isshown here mounted on its rim 9. The carcass reinforcement 7 is formedof at least one ply reinforced by so-called “radial” cables, that is tosay that these cables are arranged practically parallel to each otherand extend from one bead to the other so as to form an angle of between80° and 90° with the median circumferential plane (plane perpendicularto the axis of rotation of the tire which is located halfway between thetwo beads 4 and passes through the centre of the crown reinforcement 6).

[0112] The tire according to the invention is characterised in that itscarcass reinforcement 7 comprises at least one carcass ply, the radialcables of which are multi-layer steel cables according to the invention.

[0113] In this carcass ply, the density of the cables according to theinvention is preferably between 40 and 100 cables per dm (decimeter) ofradial ply, more preferably between 50 and 80 cables per dm, thedistance between two adjacent radial cables, from axis to axis, thusbeing preferably between 1.0 and 2.5 mm, more preferably between 1.25and 2.0 mm. The cables according to the invention are preferablyarranged such that the width (“l”) of the rubber bridge, between twoadjacent cables, is between 0.35 and 1 mm. This width l in known mannerrepresents the difference between the calendering pitch (laying pitch ofthe cable in the rubber fabric) and the diameter of the cable. Below theminimum value indicated, the rubber bridge, which is too narrow, risksmechanically degrading during working of the ply, in particular duringthe deformation which it experiences in its own plane by extension ofshearing. Beyond the maximum indicated, there are risks of flaws inappearance occurring on the sidewalls of the tires or of penetration ofobjects, by perforation, between the cables. More preferably, for thesesame reasons, the width “

” is selected between 0.5 and 0.8 mm.

[0114] Preferably, the rubber composition used for the fabric of thecarcass ply has, when vulcanized, (i.e. after curing) a secant tensilemodulus M10 which is less than 8 MPa, more preferably between 4 and 8MPa. It is in such a range of moduli that the best compromise ofendurance between the cables of the invention on one hand and thefabrics reinforced by these cables on the other hand has been recorded.

[0115] III. EXAMPLES OF EMBODIMENT OF THE INVENTION

[0116] III-1. Nature and Properties of the Wires Used

[0117] To produce the examples of cables whether or not in accordancewith the invention, fine carbon steel wires are used which are preparedin accordance with known methods such as are described, for example, inapplications EP-A-0 648 891 or WO98/41682 mentioned above, starting fromcommercial wires, the initial diameter of which is approximately 1 mm.The steel used is a known carbon steel (USA Standard AISI 1069), thecarbon content of which is approx. 0.7%, comprising approximately 0.5%manganese and 0.2% silicon, the remainder being formed of iron and theusual inevitable impurities linked to the manufacturing process for thesteel.

[0118] The commercial starting wires first undergo known degreasingand/or pickling treatment before their later working. At this stage,their tensile strength is equal to about 1150 MPa, and their elongationat break is approximately 10%. Then copper is deposited on each wire,followed by a deposit of zinc, electrolytically at ambient temperature,and then the wire is heated thermally by Joule effect to 540° C. toobtain brass by diffusion of the copper and zinc, the weight ratio(phase α)/(phase α+phase β) being equal to approximately 0.85. No heattreatment is performed on the wire once the brass coating has beenobtained.

[0119] Then so-called “final” work-hardening is effected on each wire(i.e. after the final heat treatment), by cold-drawing in a wet mediumwith a drawing lubricant which is in the form of an emulsion in water.This wet drawing is effected in known manner in order to obtain thefinal work-hardening ratio (ε), calculated from the initial diameterindicated above for the commercial starting wires.

[0120] By definition, the ratio of a work-hardening operation, ε, isgiven by the formula ε=Ln (S_(i)/S_(f)), in which Ln is the Naperianlogarithm, S_(i) represents the initial section of the wire before thiswork-hardening and S_(f) the final section of the wire after thiswork-hardening.

[0121] By adjusting the final work-hardening ratio, thus two groups ofwires of different diameters are prepared, a first group of wires ofaverage diameter φ equal to approximately 0.200 mm (ε=3.2) for the wiresof index 1 (wires marked F₁) and a second group of wires of averagediameter φ equal to approximately 0.175 mm (ε=3.5) for the wires ofindex 2 (wires marked F₂).

[0122] The steel wires thus drawn have the mechanical propertiesindicated in Table 1.

[0123] The brass coating which surrounds the wires is of very lowthickness, significantly less than one micrometer, for example of theorder of 0.15 to 0.30 μm, which is negligible compared with the diameterof the steel wires. Of course, the composition of the steel of the wirein its different elements (for example C, Mn, Si) is the same as that ofthe steel of the starting wire.

[0124] It will be recalled that during the process of manufacturing thewires, the brass coating facilitates the drawing of the wire, as well asthe gluing of the wire to the rubber. Of course, the wires could becovered with a fine metal layer other than brass, having for example thefunction of improving the corrosion resistance of these wires and/or theadhesion thereof to the rubber, for example a fine layer of Co, Ni, Zn,Al, or of an alloy of two or more of the compounds Cu, Zn, Al, Ni, Co,Sn.

[0125] III-2. Production of the Cables

[0126] The above wires are then assembled in the form of layered cablesof structure [1+6+11] for the cables according to the invention (cablesC-I), of structure [1+6+12] for the cables of the prior art (cablesC-II); the wires F₁ are used to form the core C0, and the wires F₂ toform the layers C1 and C2 of these various cables.

[0127] These cables are manufactured using cabling devices (Barmagcabler) and using processes well-known to the person skilled in the artwhich are not described here in order to simplify the description. Thecable C-II is manufactured in a single cabling operation (p₁=p₂),whereas the cable C-I, owing to its different pitches p₁ and p₂,requires two successive operations (manufacture of a [1+6] cable thencabling of the final layer around this [1+6] cable), these twooperations possibly advantageously being effected in-line using twocablers arranged in series.

[0128] The cable C-I according to the invention has the followingcharacteristics:

[0129] structure [1+6+11]

[0130] d₀=0.200;

[0131] (d₀/d₁)=1.14;

[0132] d₁=d₂=0.175;

[0133] p₁=7; p₂=10.

[0134] The control cable C-II has the following characteristics:

[0135] structure [1+6+12]

[0136] d₀=0.200;

[0137] (d₀/d₁)=1.14;

[0138] d₁=d₂=0.175;

[0139] p₁=10; p₂=10.

[0140] Whatever the cables, the wires F₂ of layers C1 and C2 are woundin the same direction of twist (Z direction). The two cables tested aredevoid of wrap and have the same diameter of approximately 0.90 mm. Thediameter d₀ of the core of these cables is the same diameter as that ofits single wire F₁, which is practically devoid of torsion on itself. Itwill be noted that these two cables are of very similar construction,the cable of the invention being distinguished solely by the fact thatits outer layer C2 comprises one wire less and that its pitches p₁ andp₂ are different, while furthermore satisfying the above relationship(v). In the cable C-I, N is less by 1 than the maximum number (hereN_(max)=12) of wires which can be wound in a single saturated layeraround the layer C1.

[0141] The cable of the invention is a cable having tubular layers asshown in cross-section in FIG. 1, which has already been commented on.The control cable is a compact layered cable as shown in FIG. 2. It canbe seen in particular from this cross-section of FIG. 2 that cable C-II,although of very similar construction, owing to its method of cabling(wires wound in the same direction and pitches p₁ and p₂ being equal)has a far more compact structure than that of cable C-I; as a result, notubular layer of wires is visible, the cross-section of this cable C-IIhaving a contour E which is no longer circular but hexagonal.

[0142] It will be noted that the cable C-I of the invention (M=6) doessatisfy the following characteristics:

[0143] (i) 0.14<d₀<0.28;

[0144] (ii) 0.12<d₁<0.25;

[0145] (iii) 0.12<d₂<0.25;

[0146] (iv) 1.10<(d₀/d₁)<1.40;

[0147] (v) 5π(d₀+d₁)<p₁<p₂<5π(d₀+2d₁+d₂);

[0148] (vi) the wires of layers C1 and C2 are wound in the samedirection of twist.

[0149] This cable C-I furthermore satisfies each of the followingpreferred relationships:

[0150] 5.3π(d₀+d₁)<p₁<p₂<4.7π(d₀+2d₁+d₂);

[0151] 0.18<d₀<0.24;

[0152] 0.16<d₁=d₂<0.19;

[0153] 5<p₁<p₂<12.

[0154] The mechanical properties of these various cables are shown inTable 2. The elongation At shown for the wires is the total elongationrecorded upon breaking of the wire, that is to say both the elasticportion of the elongation (Hooke's Law) and the plastic portion of theelongation. As for the elongation of the cables, there is added in knownmanner to these two portions the so-called structural portion of theelongation, which is inherent to the specific geometry of the cabletested.

[0155] III-3. Endurance in the Tire

[0156] A) Test 1

[0157] The above layered cables are incorporated by calendering on arubberised fabric formed of a known composition based on natural rubberand carbon black as reinforcing filler, which is conventionally used formanufacturing carcass plies for radial truck tires (M10 equal toapproximately 6 MPa, after curing). This composition essentiallycomprises, in addition to the elastomer and the reinforcing filler, anantioxidant, stearic acid, an extending oil, cobalt naphthenate asadhesion promoter, and finally a vulcanization system (sulphur,accelerator, ZnO).

[0158] These cables are arranged parallel in known manner at a densityof 63 cables per dm of ply, which, taking into account the diameter ofthe cables, is equivalent to a width “l” of the rubber bridges, betweentwo adjacent cables, of approximately 0.70 mm.

[0159] Then two series (referenced P-1 and P-2) of truck tires, ofdimension 315/80 R 22.5 XZA, are manufactured which are intended to bemounted on a rim having conical seats (inclination of 15 degrees) withtwo tires in each series, one intended for running, and the other fordecortication on a new tire. The carcass reinforcement of these tires isformed of a single radial ply formed of the rubberised fabric above,reinforced by cables C-I and C-II respectively.

[0160] The tires P-1 constitute the series in accordance with theinvention, and tires P-2 the control series of the prior art. Thesetires are therefore identical with the exception of the layered cableswhich reinforce their carcass reinforcement 7.

[0161] Their crown reinforcement 6, in particular, is in known mannerformed of (i) two triangulation half-plies reinforced with metal cablesinclined at 65 degrees, surmounted by (ii) two crossed superposedworking plies, reinforced with inextensible metal cables which areinclined at 26 degrees (radially inner ply) and 18 degrees (radiallyouter ply), these two working plies being covered by (iii) a protectivecrown ply reinforced with elastic metal cables (high elongation)inclined at 18 degrees. In each of these crown reinforcement plies, themetal cables used are known conventional cables, which are arrangedsubstantially parallel to each other, and all the angles of inclinationindicated are measured relative to the median circumferential plane.

[0162] The tires P-2 are tires sold by the Applicant for heavy vehiclesand, owing to their recognised performance, constitute a control ofchoice for this test.

[0163] These tires are subjected to a running test as described in §1-3,with a total of 250,000 km covered. The distance imposed on each type oftire is very great; it is equivalent to continuous running of a durationof approximately five months and to 80 million fatigue cycles.

[0164] Despite these very severe running conditions, the two tirestested run without damage until the end of the test, in particularwithout breaking of the cables of the carcass ply; this illustrates inparticular for the person skilled in the art the high performance of thetwo types of tires, including the control tires.

[0165] After running, decortication is effected, that is to sayextraction of the cables from the tires. The cables are then subjectedto tensile tests, by measuring each time the initial breaking load(cable extracted from the new tire) and the residual breaking load(cable extracted from the tire after running) of each type of wire,according to the position of the wire in the cable, and for each of thecables tested. The average degeneration ΔFm given in % in Table 3 iscalculated both for the core wires (C0) and for the wires of layers C1and C2. The overall degenerations ΔFm are also measured on the cablesthemselves.

[0166] On reading Table 3, it will be noted that, whatever the zone ofthe cable which is analysed (core C0, layers C1 or C2), the best resultsare recorded on the cable C-I according to the invention. Although thedegenerations ΔFm remain fairly similar as far as the outer layer C2 isconcerned (although less in the cable according to the invention), itwill be noted that the further one penetrates into the cable (layer C1then core C0), the more the gaps become in favor of the cable accordingto the invention; the degeneration of the core, in particular, is fourtimes less in the cable of the invention (2% instead of 8%). The overalldegeneration ΔFm of the cable of the invention is substantially lessthan that of the control cable (2% instead of 5%).

[0167] Correlatively to the above results, visual examination of thevarious wires shows that the phenomena of wear or fretting (erosion ofmaterial at the points of contact), which result from repeated frictionof the wires on each other, are substantially reduced in the cables C-1compared with the cables C-2.

[0168] These results are unexpected given that the person skilled in theart might expect, on the contrary, that the selection of different helixpitches p₁ and p₂ in the cable according to the invention, and hence thepresence of different angles of contact between the layers C1 and C2—theeffect of which is to reduce the contact surfaces and hence to increasethe contact pressures between the wires of layers C1 and C2—would on thecontrary result in an increase in the friction and hence the wearbetween the wires, and finally, ultimately will adversely affect thecable. Such is not the case.

[0169] B) Test 2

[0170] A new running test is performed with the same fabrics aspreviously, reinforced with the cables C-I and C-II, by manufacturingtwo other series of tires (two tires per series), of the same dimensionsas in the preceding test. The tires according to the invention arereferenced P-3, and the control tires are referenced P-4. The specificconditions of the running test are the same as previously, with thedifference that the distance travelled is increased further by 50,000km, i.e. a total of 300,000 km imposed on the tires.

[0171] The results of Table 3 confirm the results of Test 1 above. Thelowest degenerations are once again recorded on the cable C-I accordingto the invention (tire P-3), whatever the layer in question. The furtherone penetrates into the cable, the more the gaps become to the benefitof the cable of the invention, with in particular a degeneration of thecore which is four times less than in the case of the control cable (3%instead of 12%). The result is an overall degeneration of the cable ofthe invention which is substantially less than that of the control cable(4% instead of 7%).

[0172] C) Test 3

[0173] A new running test is performed with the same rubberised fabricsas before, but this time in truck tires intended to be mounted on aflat-seat rim, of dimension 10.00 R 20 XZE.

[0174] All the tires tested are identical, with the exception of thelayered cables which reinforce their carcass reinforcement 7. Thiscarcass reinforcement 7 is formed of a single radial ply formed of therubberised fabric above, reinforced either by cables C-I or by cablesreferenced C-III. The crown reinforcement 6 of these tires is in knownmanner formed of (i) two crossed superposed working plies, reinforcedwith metal cables inclined by 22 degrees, these two working plies beingcovered by (ii) a protective crown ply reinforced by elastic metalcables inclined at 22 degrees. In each of these crown reinforcementplies, the metal cables used are known conventional cables, which arearranged substantially parallel to each other, and all the angles ofinclination indicated are measured relative to the mediancircumferential plane.

[0175] A series of two tires (referenced P-5) is reinforced by thecables C-I, and another series of two tires (referenced P-6) isreinforced by known cables (referenced C-III) which are describedhereafter. In each series, one tire is intended for running, and theother for decortication on a new tire. The tires P-5 thereforeconstitute the series in accordance with the invention, and tires P-6the control series.

[0176] The cables C-III are known cables of wrapped structure [3+9](0.23), commonly used for reinforcing truck tires of such dimensions.They are formed of 12 wires (referenced F₃ in Table 4) of the samediameter 0.23 mm, with a core of 3 wires wound together in a helix (Sdirection) in a pitch of 6.5 mm, this core being surrounded by a singlelayer of 9 wires which themselves are wound together in a helix (Sdirection) in a pitch of 12.5 mm; the assembly is wrapped by a singlewire of diameter 0.15 mm wound in a helix (Z direction) at a pitch of3.5 mm. The 12 wires (referenced F₃) and the cable (referenced C-III)have the properties indicated in Table 4.

[0177] These tires are subjected to a severe running test such as isdescribed in section I-3, but this time with the test being performeduntil destruction of one of the tires tested occurs.

[0178] It will be noted that the control tire P-6, under the forcedconditions of running which are imposed thereon, is destroyed at the endof 100,000 km, following breaking of the carcass ply (numerous cablesC-III broken). Running is then stopped on the tire P-5 according to theinvention, then the cables of the invention are extracted to measure thedegeneration ΔFm of their breaking load. It will then be noted that thecables C-I according to the invention all resisted the running test (nobreaking) and that they underwent an average breaking-load loss ΔFmwhich remains relatively low since it is less than 10% (8% on the cable,7 to 9% on the wires taken individually, depending on the layeranalysed). The use of the cable according to the invention thereforemakes it possible quite significantly to increase the life of thecarcass, which is moreover already excellent for the control tire.

[0179] D) Air Permeability Test

[0180] The endurance results described previously appear to be wellcorrelated to the amount of penetrability of the cables by the rubber,as explained hereafter.

[0181] The non-fatigued cables C-I to C-III (after extraction from thenew tires) were subjected to the air permeability test described insection I-2, by measuring the amount of air passing through the cablesin 1 minute (average of 10 measurements). The permeability indices Paobtained are set forth in Table 5 (in relative units). The three valuesindicated correspond to samples taken at three different points of thecarcass reinforcement of the tires (shoulder, mid-sidewall and bottomzone of the tire), the base 100 being used for the control cables C-IIof structure [1+6+12] which are used in the tires P-2 and P-4.

[0182] It will be noted that the cable according to the invention is theone which, by very far, has the lowest air permeability index Pa (10times lower than that of the control C-II, and practically 30 timeslower than that of the control cable C-III), and hence the highestamount of penetration by the rubber. Its specific construction makes itpossible, during the moulding and/or curing of the tires, for virtuallycomplete migration of the rubber within the cable to occur, as far asthe center of the latter, without forming empty channels. The cable,which is thus rendered impermeable by the rubber, is protected from theflows of oxygen and moisture which pass, for example, from the sidewallsor the tread of the tires towards the zones of the carcassreinforcement, where the cable, in known manner, is subjected to themost intense mechanical working.

[0183] III-4. Other Comparative Tests

[0184] In this new series of tests, 5 layered cables are prepared,referenced C-IV to C-VIII, of construction (1+6+11) or (1+7+11), thesecables being or not being in accordance with the invention, in orderthen to subject them to the undulating-traction fatigue test describedin section I-5 above.

[0185] A) Test 4 (Cables C-IV to C-VI)

[0186] Cables C-IV to C-VI, prepared from the wires F₁ and F₂ describedabove, have the properties indicated in Table 6 and the followingcharacteristics.

[0187] Cable C-IV (according to the invention):

[0188] structure [1+6+11]

[0189] d₀=0.200;

[0190] (d₀/d₁)=1.14;

[0191] d₁=d₂=0.175;

[0192] p₁=7; p₂=10.

[0193] Cable C-V (control):

[0194] structure [1+6+11]

[0195] d₀=0.200;

[0196] (d₀/d₁)=1.14;

[0197] d₁=d₂=0.175;

[0198] P₁=5; p₂=10.

[0199] Cable C-VI (control):

[0200] structure [1+6+11]

[0201] d₀=0.200;

[0202] (d₀/d₁)=1.14;

[0203] d₁=d₂=0.175;

[0204] p₁=7.5; p₂=15.

[0205] These cables therefore have a very similar construction: in thethree cases, N is less by 1 than the maximum number (here N_(max)=12) ofwires which can be wound in a single saturated layer around the layerC1; they all have a tubular-layer construction as shown in FIG. 1; thepitches p₁ and p₂ are different for each cable; the pitches p₂ arefurthermore identical for cables C-IV and C-V. However, only cable C-IVsatisfies the above relationship (v) and is therefore in accordance withthe invention.

[0206] In the undulating-traction fatigue test, these three cablesyielded the results appearing in Table 7; σ_(d) y is expressed in MPaand in relative units (r.u.), the base 100 being used for the cable ofthe invention.

[0207] It will be noted that, unexpectedly, despite the very similarconstructions of the cables, the cable of the invention C-IV isdistinguished by significantly greater fatigue strength than that of thetwo control cables, in particular greater by 23% than that of thecontrol cable C-V, only the pitch p₁ of which differs (relationship (v)not satisfied) in comparison with the cable of the invention.

[0208] B) Test 5 (Cables C-VII and C-VIII)

[0209] Cables C-VII and C-VIII were prepared starting from, on one hand,the wires F₂ previously described (average diameter φ equal to 0.175 mm)for their layers C1 and C2, and, on the other hand, from the wiresreferred to hereafter as F₄ (average diameter 4 equal to approximately0.250 mm) for their core C0. These wires F₄ were manufactured asindicated in section III-1 above, by adjusting the final work-hardeningratio to obtain the intended final diameter; they have the mechanicalproperties indicated in Table 8.

[0210] These cables C-VII and C-VIII have the following properties (seemechanical properties in Table 8):

[0211] Cable C-VII (according to the invention):

[0212] structure [1+7+11]

[0213] d₀=0.250;

[0214] (d₀/d₁)=1.43;

[0215] d₁=d₂=0.175;

[0216] p₁=7;p₂=11.

[0217] Cable C-VIII (control):

[0218] structure [1+7+11]

[0219] d₀=0.250;

[0220] (d₀/d₁)=1.43;

[0221] d₁=d₂=0.175;

[0222] p₁=5; p₂=10.

[0223] These cables therefore have a very similar construction: in thetwo cases, N is less by 2 than the maximum number (here N_(max)=13) ofwires which can be wound in a single saturated layer around the layerC1, all these cables having a tubular layer-type structure asillustrated in FIG. 1. The pitches p₁ and p₂ are similar from one cableto the other, but only the cable C-VII satisfies the above relationship(v) and is therefore in accordance with the invention.

[0224] In the undulating-traction fatigue test (Table 9—base 100 usedfor the control cable as far as the values in relative units areconcerned), the cable of the invention C-VII is distinguished by asignificantly greater endurance π_(d) (plus 26% approximately comparedwith the control), which confirms the results of the previous test(Table 7). Furthermore, an air-permeability measurement Pa was carriedout, emphasising here too the superiority of the cable C-VII from thepoint of view of its ability to be penetrated by the rubber, and hencethe best overall compromise of properties which is available with thecable of the invention.

[0225] In conclusion, as shown by the various tests above, the cables ofthe invention make it possible to reduce significantly the phenomena offatigue-fretting corrosion in the carcass reinforcements of the tires,in particular the truck tires, and thus to improve the longevity ofthese tires.

[0226] Of course, the invention is not limited to the examples ofembodiment described above.

[0227] Thus, for example, the core C0 of the cables of the inventionmight be formed of a wire of non-circular section, for example, onewhich is plastically deformed, in particular a wire of substantiallyoval or polygonal section, for example triangular, square oralternatively rectangular; the core C0 might also consist in a preformedwire, whether or not of circular section, for example an undulating orcorkscrewed wire, or one twisted into the shape of a helix or a zigzag.In such cases, it should of course be understood that the diameter d₀ ofthe core represents the diameter of the imaginary cylinder of revolutionwhich surrounds the core wire (diameter of bulk), and not the diameter(or any other transverse size, if its section is not circular) of thecore wire itself. The same would apply if the core C0 were formed not ofa single wire as in the above examples, but of several wires assembledtogether, for example two wires arranged parallel to each other oralternatively twisted together, in a direction of twist which may or maynot be identical to that of the intermediate layer C1.

[0228] For reasons of industrial feasibility, cost and overallperformance, it is however preferred to implement the invention with asingle conventional linear core wire, of circular section.

[0229] Furthermore, since the core wire is less stressed during thecabling operation than the other wires, bearing in mind its position inthe cable, it is not necessary for this wire to use, for example, steelcompositions which offer high ductility in torsion; advantageously, anytype of steel could be used, for example a stainless steel, in order toresult, for example, in a hybrid steel [1+6+11] cable such as describedin the aforementioned application WO98/41682, comprising a stainlesssteel wire at the centre and 17 carbon steel wires around it.

[0230] Furthermore, (at least) one linear wire of one of the two layersC1 and/or C2 might also be replaced by a preformed or deformed wire, ormore generally by a wire of section different from that of the otherwires of diameter d₁ and/or d₂, so as, for example, to improve stillfurther the ability of the cable to be penetrated by the rubber or anyother material, the diameter of bulk of this replacement wire possiblybeing less than, equal to or greater than the diameter (d₁ and/or d₂) ofthe other wires constituting the layer (C1 and/or C2) in question.

[0231] Without modifying the spirit of the invention, all or part of thewires constituting the cable according to the invention might beconstituted of wires other than steel wires, whether metallic or not, inparticular wires of inorganic or organic material having a highmechanical strength, for example monofilaments of liquid-crystal organicpolymers such as described in Application WO92/12018. The invention alsorelates to any multi-strand steel cable (“multi-strand rope”), thestructure of which incorporates, at least, as the elementary strand, alayered cable according to the invention. TABLE 1 Wires φ Fm (N) At (%)Rm (MPa) F₁ 0.200 81.7 1.8 2718 F₂ 0.175 62.3 2.1 2856

[0232] TABLE 2 Cable Fm (N) At (%) Rm (MPa) C-I 1173 2.7 2696 C-II 12552.8 2750

[0233] TABLE 3 ΔFm (%) Tire Cable C0 C1 C2 Cable P-1 C-I 2 2 3 2 P-2C-II 8 6 5 5 P-3 C-I 3 3 4 4 P-4 C-II 12  8 5 7

[0234] TABLE 4 Wire or cable Fm (N) At (%) Rm (MPa) F₃  113 1.8 2790C-III 1310 3.3 2560

[0235] TABLE 5 Cable Pa (r.u.) Average Pa C-I  10-17-6   11 C-II102-97-102 100 C-III 315-280-305 300

[0236] TABLE 6 Cable Fm (N) At (%) Rm (MPa) C-IV 1193 2.4 2661 C-V 11812.5 2614 C-VI 1211 2.4 2702

[0237] TABLE 7 Cable σ_(d) (MPa) σ_(d) (r.u.) C-IV 765 100  C-V 621 81C-VI 676 88

[0238] TABLE 8 Wire or cable Fm (N) At (%) Rm (MPa) F₄  139 2.0 2824C-VII 1312 2.4 2667 C-VIII 1275 2.5 2570

[0239] TABLE 9 Cable σ_(d) (MPa) σ_(d) (r.u.) Pa (r.u.) C-VII 779 126 38 C-VIII 619 100 100

We claim:
 1. A multi-layer cable having a unsaturated outer layer,usable as a reinforcing element for a tire carcass reinforcement,comprising a core of diameter d₀ surrounded by an intermediate layer(C1) of six or seven wires (M=6 or 7) of diameter d₁ wound together in ahelix at a pitch p₁, this layer C1 itself being surrounded by an outerlayer (C2) of N wires of diameter d₂ wound together in a helix at apitch p₂, N being less by 1 to 3 than the maximum number N_(max) ofwires which can be wound in one layer about the layer C1, this cablehaving the following characteristics (d₀, d₁, d₂, p₁ and p₂ in mm): (i)0.14<d₀<0.28; (ii) 0.12<d₁<0.25; (iii) 0.12<d₂<0.25; (iv) for M=6:1.10<(d₀/d₁)<1.40; for M=7: 1.40<(d₀/d₁)<1.70; (v)5π(d₀+d₁)<p₁<p₂<5π(d₀+2d₁+d₂); (vi) the wires of layers C1 and C2 arewound in the same twist direction.
 2. A cable according to claim 1, ofconstruction [1+M+N], the core of which being formed by a single wire.3. A cable according to claim 2, selected from among the cables of theconstructions [1+6+10], [1+6+11], [1+6+12], [1+7+11], [1+7+12] and[1+7+13].
 4. A cable according to claim 2, of construction [1+6+N].
 5. Acable according to claim 3 or 4, of construction [1+6+11].
 6. A cableaccording to claim 1, which satisfies the following relationships:d₁=d₂; 5<p₁<p₂<15.
 7. A cable according to claim 6, which satisfies thefollowing relationships: 0.18<d₀<0.24; 0.16<d₁=d₂<0.19; 5<p₁<p₂<12.
 8. Acable according to claim 1, characterised in that it is a steel cable.9. A cable according to claim 8, characterised in that the steel is acarbon steel.
 10. A cable according to claim 1, which satisfies therelationship: 5.3π(d ₀ +d ₁)<p ₁ <p ₂<4.7π(d ₀+2d ₁ +d ₂).
 11. A trucktire having a carcass reinforcement comprising a multi-layer cablehaving a unsaturated outer layer, comprising a core of diameter d₀surrounded by an intermediate layer (C1) of six or seven wires (M=6 or7) of diameter d₁ wound together in a helix at a pitch p₁, this layer C1itself being surrounded by an outer layer (C2) of N wires of diameter d₂wound together in a helix at a pitch p₂, N being less by 1 to 3 than themaximum number N_(max) of wires which can be wound in one layer aboutthe layer C1, said multi-layer cable having the followingcharacteristics (d₀, d₁, d₂, p₁ and p₂ in mm): (i) 0.14<d₀<0.28; (ii)0.12<d₁<0.25; (iii) 0.12<d₂<0.25; (iv) for M=6: 1.10<(d₀/d₁)<1.40; forM=7: 1.40<(d₀/d₁)<1.70; (v) 5π(d₀+d₁)<p₁<p₂<5π(d₀+2d₁+d₂); (vi) thewires of layers C1 and C2 are wound in the same twist direction.
 12. Atire according to claim 11, wherein the multi-layer cable, ofconstruction [1+M+N], has a core formed by a single wire.
 13. A tireaccording to claim 12, wherein the multi-layer cable is selected fromamong the cables of the constructions [1+6+10], [1+6+11], [1+6+12],[1+7+11], [1+7+12] and [1+7+13].
 14. A tire according to claim 12,wherein the multi-layer cable has a construction [1+6+N].
 15. A tireaccording to claim 13 or 14, wherein the multi-layer cable has aconstruction [1+6+11].
 16. A tire according to claim 11, wherein thefollowing relationships are satisfied: d₁=d₂; 5<p₁<p₂<15.
 17. A tireaccording to claim 16, wherein the following relationships aresatisfied: 0.18<d₀<0.24; 0.16<d₁=d₂<0.19; 5<p₁<p₂<12.
 18. A tireaccording to claim 11, characterised in that the layer-cable is a steelcable.
 19. A tire according to claim 18, characterised in that the steelis a carbon steel.
 20. A tire according to claim 11, wherein thefollowing relationships are satisfied: 5.3π(d ₀ +d ₁)<p ₁ <p ₂<4.7π(d₀+2d ₁ +d ₂).
 21. A composite fabric usable as a carcass reinforcementply for a truck tire, comprising a matrix of rubber compositionreinforced by a multi-layer cable having a unsaturated outer layer,comprising a core of diameter d₀ surrounded by an intermediate layer(C1) of six or seven wires (M=6 or 7) of diameter d₁ wound together in ahelix at a pitch pi, this layer C1 itself being surrounded by an outerlayer (C2) of N wires of diameter d₂ wound together in a helix at apitch p₂, N being less by 1 to 3 than the maximum number N_(max) ofwires which can be wound in one layer about the layer C1, saidmulti-layer cable having the following characteristics (d₀, d₁, d₂, p₁and p₂ in mm): (i) 0.14<d₀<0.28; (ii) 0.12<d₁<0.25; (iii) 0.12<d₂<0.25;(iv) for M=6: 1.10<(d₀/d₁)<1.40; for M=7: 1.40<(d₀/d₁)<1.70; (v)5π(d₀+d₁)<p₁<p₂<5π(d₀+2d₁+d₂); (vi) the wires of layers C1 and C2 arewound in the same twist direction.
 22. A fabric according to claim 21,wherein the multi-layer cable is of construction [1+M+N], having a coreformed by a single wire.
 23. A fabric according to claim 22, wherein themulti-layer cable is selected from among the cables of the constructions[1+6+10], [1+6+11], [1+6+12], [1+7+11], [1+7+12] and [1+7+13].
 24. Afabric according to claim 22 or 23, wherein the multi-layer cable has aconstruction [1+6+N].
 25. A fabric according to claim 24, wherein themulti-layer cable has a construction [1+6+11].
 26. A fabric according toclaim 21, wherein the following relationships are satisfied: d₁=d₂;5<p₁<p₂<15.
 27. A fabric according to claim 26, wherein the followingrelationships are satisfied: 0.18<d₀<0.24; 0.16<d₁=d₂<0.19; 5<p₁<p₂<12.28. A fabric according to claim 21, wherein the cable density, in therubber composition matrix, is between 40 and 100 cables per dm offabric.
 29. A fabric according to claim 21, wherein the width l of thebridge of rubber composition, between two adjacent cables, is between0.35 and 1 mm.
 30. A fabric according to claim 29, wherein the width lof the bridge of rubber composition, between two adjacent cables, isbetween 0.5 and 0.8 mm.
 31. A fabric according to claim 30, wherein therubber composition has, in the vulcanized state, a secant tensilemodulus M10 which is less than 8 MPa.
 32. A fabric according to claim31, wherein the rubber composition has, in the vulcanized state, asecant tensile modulus M10 which is between 4 and 8 MPa.
 33. A trucktire having a carcass reinforcement comprising, as reinforcing ply, acomposite fabric according to claim
 21. 34. A truck tire having acarcass reinforcement comprising, as reinforcing ply, a composite fabricaccording to claim
 28. 35. A truck tire having a carcass reinforcementcomprising, as reinforcing ply, a composite fabric according to claim 29or
 30. 36. A truck tire having a carcass reinforcement comprising, asreinforcing ply, a composite fabric according to claim 31 or
 32. 37. Thecable of claim 1, wherein the core is comprised of L wires, wherein L isequal to or greater than 2.